An echography apparatus is an apparatus for examining objects which utilizes ultrasonic radiation as an information source. The operation of such an apparatus involves a transmission step during which ultrasonic signals are periodically applied to the medium being examined, as well as a receiving and processing step for the echos returned by the obstacles encountered in the object being examined. The two steps are executed by means of the same ultrasonic probe in contact with the object. This probe consists of a structure which is generally composed of a complete array of ultrasonic transducers.
During the transmission step the object is selectively scanned along a line. During reception, the image of the scanned line is formed, taking into account the transit time in the object and the amplitude of the echos stemming from various obstacles encountered along the line. The image of a slice is formed by scanning of such a line. In order to obtain a suitable image resolution, attempts are made to scan the object in a very selective manner by way of a focused ultrasonic activation and, upon reception, to select the echos stemming from the same line by using a focusing aperture.
A contemporary focusing technique comprises the use of a linear array of transducers and of defining, upon transmission, a focused incident beam by means of a delay rule imposed on the transducer excitation pulses. Upon reception, focusing is realized in a similar manner by suitably delaying the signals received by each of the transducers of the array before carrying out the summing and further processing of these signals. This processing of the signals in the receiving mode, resulting in a signal having a high amplitude for the echos stemming from the focusing point (which point is situated on the line scanned) and in weak signals for all other echos, is customarily referred to as "channeling". In order to achieve suitable focusing for a complete line, contemporary echography apparatus utilizes a focusing rule in the receiving mode which varies quickly as a function of time so as to be adapted to the echos received at any instant. This possibility arises from the existence of a univocal relation between the time and the depth in echography so that the echos returned by remoter structures arrive later. Actually, for the sake of simplicity, the variation of the receiving focus generally is not continuous but varies in zones. The determination of focusing delay rules is realised by application of various geometrical considerations, notably by assuming that the propagation speed of sound is constant in the object being scanned. For efficient focusing, the delays applied should also have a precision of one eighth of the wavelength; for example, for a probe having a central frequency 5 MHz, the delays must be variable in steps of 25 ns (=200 ns/8).
The use of linear arrays of transducer elements enables not only focusing but also the scanning necessary for forming a two-dimensional image. Scanning can be performed in at least three different ways which are described with reference to FIG. 1a to 1d which show different types of probe with the respective acoustic apertures A used, the axes B of the beams formed, and the boundaries D of the field examined.
The first and most simple solution (see FIG. 1a) comprises the use of an array of one hundred (or more) transducers (currently, 128 transducers are used). Thus, a transmission/reception aperture is defined which has a given width (typically from 16 to 64 elements) and which focuses along an axis extending perpendicularly to the array in the transmission mode as well as the receiving mode. Scanning is realized by displacing the aperture by analog multiplexing. Two successive apertures thus defined have an intersection which constitutes the entire aperture except for one or a few transducer elements. This mode of operation enables images to be obtained of a zone of the object situated in the geometrical shade of the probe.
According to the other two scanning techniques, all elements of the probe are used for all lines of the image. These probes at present comprise 64 or 128 elements. In one of these two modes (see FIG. 1b) the focusing rules are calculated so as to form lines perpendicularly to the array. In this case the zone of the object scanned is again situated in the shade of the probe. The importance of this mode resides in the use of all elements, that is to say large apertures, enabling a higher resolution to be achieved.
The other mode (see FIG. 1c) is used notably to obtain images of zones which are larger than the imprint of the probe on the object. The delay rules for transmission and reception are calculated so that the ultrasonic beam can enclose an arbitrary angle with respect to the axis of the probe. The image is obtained by the scanning of an angular sector (commonly from -45 to +45 degrees). Such systems, referred to as phased arrays, notably enable images to be formed of the heart in medical applications, i.e. through the acoustic window formed by one of the intervals between the sides.
Other types of probe also exist, which types utilize, for example curved linear arrays (see FIG. 1d) capable of emitting ultrasonic beams perpendicularly to their tangent, thus also enabling an image to be obtained of the object in a zone which is larger than their imprint.
The purpose of an echography apparatus being the formation of images, processing of the signals obtained after the channeling is necessary. This operation takes place in two principal steps: on the one hand, an envelope detection which extracts the amplitude information from the signal formed, and on the other hand a scan conversion which reconstructs the image on the basis of time information contained in the envelopes of the signals and the position of each line relative to the probe.
This general summary of the operation of a conventional echography apparatus enables definition of the general structure as proposed in FIG. 2a. The echography apparatus shown therein comprises first of all an array of m ultrasonic transducers 10a to 10m which are connected to an analog pointer 15 which enables definition of the aperture. The other extremity of this pointer 15 is connected on the one hand to a transmission stage 20 and on the other hand to a receiving and processing stage 30.
Generally speaking, the transmission stage 20 comprises the following elements:
(a) a sequencer circuit 21 which defines the rhythm of the ultrasonic activations with a recurrent frequency in the order of, for example from 3 to 5 kHz, and which comprises essentially an oscillator and a frequency divider which supplies the necessary different clock signals; PA1 (b) an excitation signal transmitting circuit 22 which is connected to the output of the sequencer circuit 21 and which serves to transmit electrical signals for excitation of the transducers, which excitation is controlled either according to a time rule appropriate to enable focusing of the ultrasonic signals, or is phase controlled, the different focusing delays thus being obtained (which is the case shown in FIG. 2a) by means of n delay lines 23a to 23n which are connected to the output of the circuit 22, or in the n transmission channels associated with the n transducers used for the transmission (n smaller than m), respectively; PA1 (c) if the focusing is not realized by the circuit 22 (as indicated sub (b)), the n delay lines 23a to 23n constitute an electronic focusing circuit 23; PA1 (d) a circuit 24 for activating the high voltage, delivering the high-voltage pulses for execution of the transmission by the transducers. PA1 (a) preamplifiers 31a to 31n, the group of n preamplifiers constituting a preamplifier circuit 31 which receives the n echographic signals corresponding to the aperture of the transducer array; PA1 (b) a circuit 32 for gain compensation as a function of time; PA1 (c) delay lines 33a to 33n, the group of delay lines constituting a circuit 33 for focusing in the receiving mode (dynamic focusing). PA1 an envelope detector 43 which receives the output signal of the summing device 41 and which is followed by an analog-to-digital converter 44; PA1 a scan converter 45 which receives on the one hand the output signal of the analog-to-digital converter 44 and on the other hand the output signal of a memory 46 which defines the positions of the lines scanned with respect to the probe, and also receives synchronization signals which are again supplied by the sequencer circuit 21; PA1 an image memory 47 which serves as a buffer memory for the output signals of the scan converter 45 and whose output signals are themselves displayed on a display screen 48 after the reading of the memory.
The sequencer circuit 21 supplies not only the synchronization pulses for the ultrasonic activations, but also the control signals for a delay rule memory circuit 25 for controlling focusing in the transmission mode. This circuit 25 comprises a memory containing the sequence of delay rules in the transmission mode for each transducer, which sequence serves to achieve the configuration of the delay lines 23a to 23n of the focusing circuit 23 according to a predefined rule for each activation.
The receiving and processing stage 30 consists first of all of n receiving and processing channels which comprise the following elements in the present embodiment:
The gain compensation circuit 32 essentially comprises n amplifiers 32a to 32n whose gain is variable as a function of time and which are controlled by a control circuit 35 which itself receives synchronization pulses from the sequencer circuit 21. The circuit 33 for focusing in the receiving mode is connected to a memory 34 which stores, for each channel, the set of delay rules for each focusing zone and for each line of the image; the memory 34 itself is also controlled by the sequencer circuit 21.
A summing device 41 thus receives the output signals of the n receiving and processing channels thus formed (actually, the output signals of the circuit 33), and is followed by known circuits which enable images of slices of the object scanned to be obtained. This group of circuits, collectively denoted by the term "processing and display sub-assembly 42" and shown in FIG. 2b, essentially comprises:
It is to be noted that this summary of the configuration of a conventional echography apparatus is a general summary and that various modifications also merit mentioning, notably those which relate to the digital processing of the signals in the receiving mode (or the formation of the channels) by inserting, for example an analog-to-digital converter at the output of the circuit 32 for gain compensation as a function of time. One improvement encountered in given high-end echography apparatus consists, as appears from the embodiment shown in FIG. 2c, in the distribution of the gain compensation as a function of time between two distinct gain compensation circuits 321 and 322; one of these circuits is arranged at the output of the circuit 31 as before and upstream from the circuit for focusing in the receiving mode in order to perform a first gain compensation which already allows for elimination of most noise, the other circuit being situated downstream from the circuits providing focusing and summing, just ahead of the sub-assembly 42, in order to impart a finer, complementary compensation.
The focusing and summing functions, however, can also be realized in a way other than described thus far by utilizing, as is shown in FIG. 2c, a pointer 36 and a single delay line 37. The pointer 36 is controlled by the sequencer 341 which is connected to the output of the circuit 21 as before and comprises as many inputs as there are transducers present in the aperture and as many outputs as there are possible delays. These outputs are connected to different input points of the delay line 37. The pointer 36 receives the output signals of the gain compensation circuit 321, converts the signals into a current and subsequently directs each signal thus formed to the output which corresponds to the desired delay. Summing is realized by natural addition of the currents entering the delay line 37.
The operating principle of contemporary echography apparatus is generally based on the hypothesis of a constant ultrasonic speed in the tissues scanned, notably to enable calculation of the various focusing delays and any angulation of the beams and to enable conversion of information concerning the transit times of echos into depth information. However, this hypothesis is rarely verified: the propagation speed of ultrasonic waves, for example has a mean value of 1540 m/s in the liver, while in lipid tissues it amounts to approximately 1300 m/s. Consequently, during transmission as well as reception defocusing of the ultrasonic beams occurs, leading to a loss of resolution and contrast of the images, which loss is higher as larger focusing apertures and probes of higher frequency are used. The effect of the frequency on the degradation thus observed can be understood by considering the necessity of conserving a precision of the delays of approximately one eight of the wavelength, corresponding to a precision which is better as the frequency is higher, while the effect of the width of the aperture can be explained by observing that as the aperture is greater, the probability that zones of different speed of sound are encountered is greater.
One solution to reduce the interference resulting from such defocusing consists in the comparison by correlation of the echographic signals received by the various transducers and by subsequently delaying each of these signals before their combination and the execution of the operations as regards envelope detection, filtering and display. The proposed operation thus consists of a single correction, in the receiving chain and after the first activation, of the delays affecting each of the echographic signals received. European Patent Application EP 0 256 481, which corresponds to U.S. Pat. No. 4,817,614, describes an ultrasonic echography apparatus comprising an adaptive focusing device which utilises such a method. The cited document also describes how the method can be carried out.